True, dial up users do get a new ip address daily, but how many dialup users are there out there?
Really the IP check doesn't prevent the scammers, it just discourages them from submitting 100 different requests that the Quality Control folks would have to look at.
To even be eligible for the second tier a sample of your work would have to be reviewed first, and once you reach the second tier if you start uploading a bunch of garbage and doing all the old scams on getting extra clicks other people can report you easily to the QC folks or a second security committee if folks feel a second group would be in order for this purpose, but they can then review your submissions and if they feel it's warranted terminate your revenue sharing access immediately.
This pretty much kills the scammers in their tracks - and your right, the paypal email address is probably the best overall verification you can get, just have the site track those who have been rejected or terminated based on paypal email addressess and not allow people to keep signing up for the second tier under mulitple accounts that way.
Basically the verification is more to reduce the workload on the QC and or security folks, it doesn't catch the scammers persee, just makes it a little harder for them to circumvent the system.